GaN-on-Si transistors are suitable for making high-efficiency and low-cost power switching module; however, the voltage breakdown performance of GaN-on-Si transistors is limited by the thickness of the III-nitride epilayer stack. It is challenging to grow III-nitride epilayer stack thicker than 5 micrometers, due to the large thermal mismatch between the III-nitride material and the Si substrate.
It is well known that the Si substrate can limit the breakdown voltage of GaN-on-Si transistors. Bin Lu and Tomas Palacios, “High Breakdown (>1500V) AlGaN/GaN HEMTs by Substrate-Transfer,” IEEE Electron Device Letters, vol. 31, no. 9, pp. 951-953, September 2010 describe the removal of the Si substrate, and the subsequent transfer of the device to a glass substrate. Devices showed higher breakdown voltage after the substrate transfer; however, a disadvantage of this approach is that it is difficult to handle large-size wafers with their substrates completely removed, resulting in low yield and high cost.
P. Srivastava, J. Das, D. Visalli, M. V. Hove, P. E. Malinowski, D. Marcon, S. Lenci, K. Geens, K. Cheng, M. Leys, S. Decoutere, R. Mertens, and G. Borghs, “Record Breakdown Voltage (2200V) of GaN DHFETs on Si with 2-μm Buffer Thickness by Local Substrate Removal,” IEEE Electron Device Letters, vol. 32, no. 1, pp. 30-32, January 2011 describe an improvement in breakdown voltage after local removal of the Si substrate between the source and the drain. A disadvantage of this approach to improving breakdown voltage is that an unfilled trench in the Si substrate may cause electron trapping problems, which affect the dynamic characteristics of the GaN FETs. In particular the devices with an etched substrate may have poor dynamic on-resistance due to trapping at the substrate/air interface.
What is needed are III-nitride transistors and a method to fabricate III-nitride transistors with improved breakdown voltage that have good performance characteristics and are cost effective to fabricate. The embodiments of the present disclosure answer these and other needs.